The Mercedes shop manual I have for this transmission essentially says not to bother with checking the vacuum modulator for harsh engagement like you describe. That only leaves high line pressure as the cause. This can be due either to high idle speed or something in the valve body causing line pressure to be too high.

The band problem you refer to is probably the B2 piston failure, but this causes no forward engagement because B2 or K2 have to be applied to get any torque through the rear gearset (except in reverse, there is a one-way clutch there that applies due to reverse input to the rear gearset) Reverse is unaffected in this case since the only apply component involved in reverse is the reverse clutch. Now, those are also known to go bad usually causing delayed engagement in reverse only.

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Erich Loepke
2010 Ford Focus
Currently Benz-less